Periscopes are optical instruments for conducting observations from a concealed or protected position. A simple periscope consists essentially of reflecting mirrors or prisms at opposite ends of a tube with the reflecting surfaces parallel to each other, and at a 45° angle to the axis of the tube. The so-called field or tank periscope has been commonly used in trenches, behind parapets and earthworks, and in tanks to provide protected vision for the user. Periscopes are also used as viewing devices in military aircraft, in nuclear physics laboratories to observe radioactive reactions, and in particle accelerators.
The physics behind periscope operation, as mentioned above, is based on principles of light reflection and has been implemented for many years. Light always reflects away from a mirror at the same angle that it hits the mirror. As mentioned previously, a simple periscope consists essentially of reflecting mirrors or prisms at opposite ends of a tube with the reflecting surfaces parallel to each other, and at a 45° angle to the axis of the tube. With this periscope configuration, light hits the top mirror at a 45° angle and reflects away at the same angle, which bounces it down to the bottom mirror. That reflected light hits the second mirror at a 45° angle and reflects away at the same angle, right into your eye. One such periscope configuration is U.S. Pat. No. 3,454,222 owned by United States of America, as represented by the Secretary of the Army. In that invention the periscope utilized a catadioptric system for night periscope sight. Another such periscope, also owned by the United States of America, as represented by the Secretary of the Army, is U.S. Pat. No. 3,549,231, a lens prescription for an optical system for day and night periscope sight. In these early periscope configurations the images are observed via the reflective characteristics of the mirrors along with their positions relative to each other. However, these earlier periscope configurations had very limited image production and night time viewing capabilities.
With the advancement of optical technology periscopes became more and more advanced as well. Night time image viewing capabilities improved, whether of the lens switching type, side by side sensor systems type, or the aperture sharing systems type; however these refractive optical systems had their disadvantages too. The lens switching system required the use of additional manpower, or bulky expensive mechanical or electromechanical lens switching mechanisms, requiring the attention of the operator for the lens selection. The side by side sensor systems required boresighting both of them on a common elevation or azimuth or both using optics such as a head mirror or a prism. The resulting structure is large, complicated, and expensive to manufacture, repair and maintain. The aperture sharing systems have increased substantially the system diameter which complicates stabilization of the system and adds to the size and the cost of the system, especially in a panoramic periscope where 360 degrees of azimuth coverage is required and the torquers, resolvers, and slip-ring assemblies become large. One such aperture sharing system with typical lens switching for covering wide chromatic bandwidths U.S. Pat. No. 4,260,217.
The assignee of the present invention, Selectron, currently sells the commander periscope, model M-36, U.S. Pat. No. 5,943,163, the disclosures of which are herein incorporated by reference, which provides a dual band periscope, which provides side-by-side imaging of an optical field of view in the visible light spectral band and the 3 to 5 micron spectral band; a 35° prism was used to accomplish this. The prism has a first portion that reflects and refracts light in the visible range. This visible light region consists of a spectrum of wavelengths, which range from approximately 700 nanometers (abbreviated nm) to approximately 400 nm; that would be 7×10−7 m to 4×10−7 m. The second portion of the prism reflects and refracts light in the infrared range of the light spectrum. Infrared light lies between the visible and microwave portions of the electromagnetic spectrum. Infrared light has a range of wavelengths, just like visible light has wavelengths that range from red light to violet. The second portion of the prism may be composed of optical grade silicon for refracting in the range of 3 microns to 5 microns. If the range of refraction desired was 8 microns to 12 microns then the second portion of the prism may be composed of optical grade germanium. A micron is the term commonly used in astronomy for a micrometer or one millionth of a meter. Portion one and portion two abut each other and are bonded at a juncture by an adhesive. The prism in that invention was allowed to pivot inside the housing so as enable the field of view to be changed.
In accordance with the invention, U.S. Pat. No. 5,943,163, visible light entering the window of the periscope is refracted and reflected by prism portion along a path into the optical viewing members contained in housing and eyepiece to provide conventional visible light observation. Infrared emissions, such as self-emissions of personnel and equipment, pass through the window and are refracted and reflected by the prism portion along infrared optical path, through the infrared focusing lens, onto an infrared detector, such as a focal plane array. The lower housing included the electronics required for processing the detected infrared image and provided a visible display of the image elements on a cathode ray tube for observation through the eyepiece. Remote viewing could also have been provided through a cable carrying the video image signal. The processing of the infrared image and display could have been controlled through control elements on the bottom of housing. The processing could have included electronic insertion of a reticule.